1. Field of the Invention
The present invention relates to Group III nitride compound semiconductors used in optical devices such as light-emitting diodes in a range of from an ultraviolet region to a red region, laser diodes, etc., and electronic devices such as high-temperature devices, etc. and particularly to methods for producing low-resistance p-type Group III nitride compound semiconductors.
2. Description of the Related Art
Generally, even in the case where p-type impurities are added to a Group III nitride compound semiconductor produced by an metal organic vapor phase epitaxy method (MOVPE method), it is impossible to obtain low-resistance p-type semiconductor crystal as obtained in a gallium phosphide (GaP) or gallium arsenide (GaAs) semiconductor which is a Group III-Group V compound semiconductor like the Group III nitride compound semiconductor. It is conceived that the cause of this fact is in that, for example, Mg (magnesium) as p-type impurities is bonded to hydrogen contained in raw material gas (such as TMG (trimethyl gallium) or ammonia) and separated at the time of crystal growth to thereby inhibit Mg from being activated to function as an acceptor even in the case where Mg takes the place of the Group III element capable of functioning as an acceptor originally in Group III nitride compound semiconductor crystal (e.g., see Non-Patent Document 1).
Therefore, in order to obtain low-resistance p-type crystal, after addition of p-type impurities, a heat treatment such as electron beam irradiation is applied to the Group III nitride compound semiconductor to thereby obtain resistivity of from 2 Ω·cm to 10 Ω·cm (hole carrier density of from about 1×1017/cm3 to about 2×1017/cm3). With respect to the reduction in resistivity due to the heat treatment, the thought that hydrogen bonded to Mg as p-type impurities is separated by heat so as to be desorbed from the inside of crystal has been reported (e.g., see Non-Patent Document 2 and Patent Documents 1 and 2).
[Non-Patent Document 1] Jpn. J. Appl. Phys., Vol. 31, pp. 1258–1266, 1992
[Non-Patent Document 2] “OYO BUTURI”, Vol. 60, p. 163, 1991
[Patent Document 1] Unexamined Japanese Patent Publication No. Hei-5-183189 (page 6, FIG. 1)
[Patent Document 2] Unexamined Japanese Patent Publication No. Hei-5-198841 (page 5, FIG. 1)
The resistivity of the p-type Group III nitride compound semiconductor is, however, still about one figure higher than that of the GaP or GaAs compound semiconductor. It cannot be said that activation of p-type impurities is achieved sufficiently. On the other hand, as shown in FIG. 12 of Unexamined Japanese Patent Publication No. Hei-8-97471, in the case of the Group III nitride compound semiconductor, as the amount of Mg added as p-type impurities increases, hole carrier density shows a tendency to increase, that is, resistivity shows a tendency to decrease before the amount of Mg reaches a certain value, but hole carrier density shows a tendency to decrease, that is, resistivity shows a tendency to increase after the amount of Mg reaches the certain value. This hole carrier density saturation phenomenon cannot be explained only by the fact that the cause of a barrier to activation of Mg as p-type impurities is bonding of hydrogen contained in the raw material gas and Mg. The present inventor conceives that a part of Mg which is p-type impurities exists as Mg substituted for Ga (gallium) which is a Group III element in the process of crystal growth of the Group III nitride compound semiconductor but the other part of Mg exists as Mg held in interstitial sites of the Group III nitride compound semiconductor, and that Mg held in the interstitial sites compensates for Mg substituted for Ga (self-compensating effect).